Conventionally, a lamination-type displacement device used for a displacement device which is employed in a positioning mechanism for an XY stage, brake, or the like has been manufactured by fabricating thin sheets of a piezoelectric ceramic material shaped in a given form, mounting electrodes on the thin sheets, polarizing them, and then bonding them together directly or via thin sheets of a metal with an organic adhesive. For devices having thin sheets using an adhesive as described above, the displacement induced by vibration of the piezoelectric device may be absorbed by the adhesive layer, depending on the working conditions. The adhesive may deteriorate at high temperatures or after the device has been used for a long time. In this way, devices of this kind have drawbacks.
For this reason, in recent years, lamination-type displacement devices fabricated by the same method as used to manufacture lamination-type chip capacitors have been put into practical use.
One example of the conventional lamination-type displacement device is constructed as shown in FIG. 18. In FIG. 18, thin sheets 41 are made of a piezoelectric ceramic material. Positive internal electrodes 42a alternate with negative internal electrodes 42b in such a way that these sheets are tightened together and stacked on top of each other to form a laminate 45. Each one fringe of the internal electrodes 42a and 42b is so formed as to be exposed. The exposed fringes are connected with external electrodes 43a and 43b extending in the direction of the lamination. Lead wires 46 are connected with the external electrodes via solder 47.
In the structure described above, when positive and negative voltages are applied to the external electrodes 43a and 43b, respectively, an electric field is set up between the internal electrodes 42a and 42b. The thin sheets 41 are elongated in the direction of thickness and produce displacement because of the longitudinal effect of the piezo-electric ceramic material. In this structure, however, the electric field strength is weaker in the marginal regions close to the side surfaces, i.e., in the regions where the internal electrodes 42a and 42b are not laid to overlap each other. Therefore, no deformation is produced. In addition, these portions serve to hinder the deformation of the whole device. Consequently, it is impossible to obtain the amount of distortion intrinsic to the electromechanical transducer material from such an alternate electrode type. Furthermore, the stress is concentrated at the boundary between the displacement portion and the non-displacement portion. If a high voltage is impressed or if a voltage is applied for a long time, the device will be destroyed.
A lamination-type displacement device which is free from the foregoing drawbacks is shown in FIG. 19. This has an improved piezoelectric displacment efficiency, and is known as a so-called total surface electrode type (for example, see Japanese Patent Laid-Open No. 196068/1983). In FIGS. 18 and 19, like components are indicated by like reference numerals. Internal electrodes 42a and 42b are formed over the whole surface of each thin sheet 1. A desired number of thin sheets are laminated in the same way as in the foregoing to form a laminate 45. At one side of the laminate 45 fabricated as described above, a coating 44a of an insulating material is formed on alternate layers, e.g., on only the internal electrodes 42b, at the ends of the internal electrodes 42a and 42b. An external electrode 43a made of an electrically conductive material is formed on the coating 44a. At the other side of the laminate 45, a coating 44b is similarly formed at the ends of the internal electrodes such as 42a not covered with the coating 44a. An external electrode 43b is formed on the coating 44b. This structure operates similarly to the structure shown in FIG. 18 except that non-displacement portions are substantially absent. Therefore, this structure produces more uniform deformation than the structure shown in FIG. 18, and less concentration of stress occurs. In consequence, a large amount of distortion intrinsic to the electromechanical transducer material can be derived. On deformation, destruction does not take place.
In the electrostrictive-effect device structure utilizing the longitudinal effect as described above, it is effective to adopt a so-called total surface electrode structure, i.e., having internal electrodes of the same cross-sectional area as that of the electrostrictive effect device, for preventing concentration of stress on generation of a distortion. In order to produce a strong electric field and a large distortion at a low voltage, it is desired to make the spacing between the internal electrodes as small as possible. In other words, it is desired to reduce the thickness of the electrostrictive material to a minimum. In the present circumstances, it is desired to make the thickness less than 100 .mu.m. However, a special contrivance is needed to electrically connect the alternate internal electrodes having the same cross-sectional area as that of the device in parallel.
In particular, in a laminate device fabricated by the same method as used to manufacture a laminate capacitor, the spacing between the neighboring electrodes is tens of microns to hundreds of microns. Moreover, the thickness of the exposed electrodes is only several microns. Therefore, it is quite difficult to bring out electrodes, or lead wires, from the alternate layers.
An electrical connection method which solves this problem is proposed in Japanese Patent Laid-Open No. 196981/1985. This method is characterized in that alternate belt-like portions of the end surfaces of the internal electrodes exposed at the side surfaces of a laminate of such an electrostrictive material are plated with a metal.
FIG. 20 is a vertical cross section of an electrostrictive-effect device connected by this method.
The method of fabricating the electrostrictive-effect device shown in FIG. 20 is first described. A laminate in which electrostrictive materials 1, 2 alternate with internal electrodes 3, 4 as shown in FIG. 21 is fabricated by applying the techniques for manufacturing laminate ceramic capacitors. The numerous internal electrodes 3 and 4 are exposed at the front and rear surfaces which are opposite side surfaces. The internal electrodes are connected with alternate layers of two temporary external electrodes 13 and 14 which are formed on the two other opposite side surfaces. This laminate and a metal plate acting as a counter electrode are placed in a plating bath. A DC voltage is applied to the temporary external electrodes 13 and 14 from this metal plate. The metal ions charged positively inside the plating bath are deposited on the internal electrodes 3 and 4. As a result, as shown in FIG. 20, the metal is deposited at 5 and 6.
Then, insulating films 7 and 8 are formed on the surface on which the metal is deposited at 5 and 6. FIG. 22 is a fragmentary horizontal cross section of the laminate having the deposited metal 5 on which the insulating film 7 is formed.
Subsequently, the insulators 7 and 8 on the deposited metal at 5 and 6 are scraped off until the metal is exposed. Then, as shown in FIG. 23, an external electrode 9 is formed on the surface on which the deposited metal 5 and the insulating film 7 are formed. Thus, alternate layers of the multiplicity of electrodes inside the device are connected together. This laminate is cut parallel to the plane containing the temporary external electrodes 13 and 14. Thus, an electrostrictive-effect device is obtained except for small portions around both ends, the external electrodes being attached to the small portions. A DC voltage is applied between the external electrodes. In this way, the device can be activated.
Generally, electrostrictive materials are sintered in oxidizing ambients and so internal electrodes are made of precious metals such as silver/palladium, platinum, or other material that is not easily oxidized. On the other hand, the plating material used for connection with an external electrode is a base metal, since it is necessary that the deposited metal be ionized. In the device of the above-described structure, therefore, if the device is placed in a high-temperature condition or other condition to form the insulating layer or external electrode, the deposited metal is oxidized. Then, the electrical conduction is hindered, or a mechanical breakage takes place. In the worst case, the insulating layer around the deposited metal cracks because of cubic expansion caused by the oxidization. The result is that the insulation resistance decreases.
In an attempt to solve the aforementioned problem, the present inventor and others formed the external electrode in a reducing ambient. However, a new problem occurred. That is, the electrostrictive material was also deoxidized. A further method may be contemplated in which a noble metal not oxidized is caused to precipitate by plating. However, this method poses problems. For example, where silver is used, the insulation resistance drops due to migration. Where platinum, palladium, or the like is employed, the electrostrictive material is corroded by the plating liquid. Also, the usage of insulating layers and external electrodes which can be formed out of organic resins at relatively low temperatures may be contemplated. Nonetheless, it is difficult to put this method into practical use, because there is a possibility that some problems take place. In particular, after the device is used for a long time, it is deteriorated. Also, the device is deteriorated by moisture. Furthermore, the mechanical strength deteriorates at high temperatures.
In the above-described method making use of metal protrusions formed by plating, the thickness of the insulating layer depends on the height of the metal protrusions formed by plating and so it is impossible to obtain an insulating layer of a sufficient thickness. Specifically, as shown in FIG. 24, let t be the thickness of an insulating layer 7. Let W be the distance between a deposited metal 5 and an internal electrode 3 which differs from the metal in electrical polarity. Increasing the distance W provides better insulation. However, if the width of the deposited metal 5 is made small in order to increase the distance W between the deposited metal 5 and the internal electrode 3, then the height of the deposited metal 5 is reduced. Hence it is impossible to make the thickness t of the insulating layer 7 have a sufficiently large value. On the other hand, as shown in FIG. 25, if the deposited metal 5 is grown sufficiently to increase the thickness t of the insulating layer 7, then the width of the deposited metal 5 increases, thus reducing the distance W between the deposited metal 5 and the internal electrode 3. Consequently, a distance sufficient for insulation cannot be obtained.
The present invention is intended to solve the foregoing problems. The invention provides a method of selectively insulating internal electrodes with high reliability.